Several mutations have been introduced into a cloned human TGF-alpha gene by site-directed mutagenesis. These mutant forms of TGF-alpha were expressed in yeast expression vectors. Some of them show altered characteristics when compared to normal (wild type) TGF-alpha. The rat TGF-alpha gene was chemically synthesized and expressed in a retrovirus vector. Infectious recombinant retrovirus carrying rat TGF-alpha makes NRK cells grow in soft agar in the presence of TGF-beta. Infected NRK cells secrete rat TGF-alpha at a higher level than most transformed cells. The rat TGF-alpha has been inserted in E. coli plasmid which has a strong promoter. The rat TGF-alpha gene has also been fused with HBsAG gene to express as fusion protein in eukaryote expression vector.